Compensating device for turbomachine bucket wheel



A nl 2, 1963 A. RUBIO ETAL COMPENSATING DEVICE FOR TURBOMACHINE BUCKETWHEEL Filed Nov. 15, 1961 CHARLES R. ERNEST JR.,

THEIR ATTORNEY.

United States Patent C F 3,084,343 CGMPENSATENG DEVHCE FGR TURBGMACHINEEUCKET WHEEL Ahdon Rubin, Schenectady, and Charles R. Ernest, .lra,

Esperance, N.Y., assignors to General Electric Company, a corporation ofNew York Filed Nov. 15, 1961, Ser. No. 152,387 4 Qiaims. (QR. 253-77)This invention relates to a compensating device for turbine wheelscarrying a number of blades thereon subject to scale formation oroxidation between blade bases, and more particularly to a device forcompensating for a phenomenon in turbines known as arch binding.

In a steam turbine, the bladed elements or buckets are often mounted onthe rim of a turbine wheel in a fashion such that the buckets areradially inserted one at a time at a specified location on the rim, andthen are slid circumferentially in dovetail mounting grooves in the rimuntil there is a full circumferential row of buckets on the rim. Withsuch a construction, the dovetailed base portions of the buckets oftenhave planar faces lying in a radial plane which abut similar faces ofadjacent buckets, so that each bucket is held circumferentially in placeby buckets pressing against it on either side thereof. With thisconstruction, it is desirable to have a tight structure in order toassure the correctness of the assembly, to determine the naturalvibration frequencies, and to prevent any looseness which may lead tounbalance. The aforedescribed type of turbine wheel construction is,however, subject to a phenomenon known as arch binding which causes agradual increase in the diameter of the wheel to which the buckets areattached.

While We do not wish to be limited to any theory, it is suggested thatarch binding takes place in the following manner. Since [the rates ofthermal expansion of the buckets and the turbine wheel are different,due to their being made of different materials, and since the wheel andbuckets are exposed to radial thermal gradients, a change in temperaturecan cause spaces to open up between adjacent faces of the bucket bases.Such an opening is also caused by the centrifugal stresses. Steam isthrottled through the spaces which can cause oxidation or leave adeposition of scale, resulting in a minute incremental increase in thecircumferential dimension of the bucket base. This increase, whenmultiplied by the number of buckets in a row, causes an effectivecircumference for the ring of buckets, which is greater than that of thecircumference of the mounting grooves available on the turbine wheel.The buckets obtain the required circumference by stretching the turbinewheel radially as temperatures are equalized. During increasingtemperature, where the wheel is colder than the buckets, the archbinding condition is magnified and the rim of the wheel may be furtherstretched radially.

This condition is cumulative and the same process occurs over again,with each new cycle causing further radial stretching of the wheel. Thestresses caused by the arch binding phenomenon can be of a magnitudegreat enough to exceed the yield and rupture strength of the wheel,thereby causing failure.

Accordingly, one object of the present invention is to provide a devicecompensating for cumulative circumferential growth of the buckets on aturbine wheel.

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Another object of the invention is to provide a device for preventingradial stretching of the turbine wheel due to arch binding of theturbine buckets.

The subject matter which is regarded as the invention is particularlypointed out and distinctly claimed in the concluding portion of thespecification. The invention, however, both as to organization andmethod of practice, together with further objects and advantagesthereof, may best be understood by reference to the followingdescription, taken in connection with the accompanying drawing in which:

FIG. 1 is a view, looking in the axial direction, of a portion of aturbine wheel with attached buckets;

FIG. 2 is a view looking in a circumferential direction of a portion ofthe arch binding compensating device;

FIG. 3 is an enlarged view, in section, illustrating the operation ofthe device; and

FIGS. 4-6 are views corresponding to FIGS. 1-3 respectively andillustrating a modification of the invention.

Briefly stated, the invention is practiced by providing a non-resilientseparator arranged to yield at a predetermined compressivecircumferential force exerted thereon so as to compensate for scaleaccumulation or oxidation between bucket base interfaces so as to reducethe effect of arch binding.

Referring now to FIG. 1 of the drawing, a plurality of turbine buckets 1have bladed portions 2 and dovetail base portions 3. The buckets 1 aredisposed on the rim-4 of a turbine wheel, a portion of which isindicated at 5. The rim 4 includes circumferential radially-spaced ribs4a, 4b forming arcuate dovetail retaining means for the bucket bases 3.Each of the bucket bases 3 has opposite planar faces 3a, which abut oneanother along a radial interface indicated at 6. When the buckets l areproperly assembled on the turbine wheel 5, there is no clearance at theinterfaces 6. It will be apparent that, under thermal transients, thedifferent rates of expansion and contraction between the buckets 1 andthe wheel 5 can cause gaps to open at the interfaces 6. Steam can 1 flowinto these spaces and leave deposits or cause oxidation.

According to the invention, a special non-resilient compensating device7 is employed which comprises a retaining block 8 holding a pluralityofpins 9.

The construction of the retaining block 8 can be seen by reference toFIG. 2. The block 8 comprises mating halves 8a, 8b held in place on rim4 by suitable means such as rivets 10. The retaining block 8 includesdovetail grooves 11 which mate with arcuatte ribs 4a, 4b on the rim 4 ofthe turbine wheel. Circumferentially directed holes 12 form seats forpins 9.

Reference to FIG. 3 of the drawing illustrates that the diameter of eachof the pins 9 is slightly greater than the diameter of the hole 12 sothat pin 9 fits with a negative clearance, as illustrated by theexaggerated sectional drawing. A considerable force is required to pushpin 9 deeper into hole 12 and, in so doing, it will be apparent that pin9 is actually crushed or extruded into hole 12. The materials andclearances selected will, of course, vary with the application, but toillustrate the order of magnitude, a force on the order of 1 ton may berequired to extrude pin 9* deeper into hole 12.

It will be apparent that when the compressive force on the end of pin 9is such that the yield point of the material of pin 9 is no longerexceeded, pin 9 will simply remain where it is. Therefore, cyclicalre-application of a predetermined compressive force on pin 9 willcontinue to shorten the circumferential dimension of the compensatingdevice 7 in incremental steps, rather than continue to build up agreater and greater reactive force as would be the case with a resilientmember.

The operation of the emobdiment disclosed in FIGS. 1-3 is as follows.When the interfaces 6 between bucket bases 3 open up to allow oxidationdeposition of steam scale during thermal transients, the circumferentialdimension of each bucket will increase slightly. Although each bucketmay become enlarged over a period of time by only a few thousandths ofan inch, the net growth of 60- to 90 buckets will constitute aconsiderable circumferential growth. As the temperature is equalized andthe gaps between bucket bases close, a compressive circumferential forcebetween buckets will provide a radial compo nent due to the taper on thebases which, without the invention, could stretch the wheel radially.However, by employing the compensating device 7, this compressive forceacts on the pins 9 to cause them to extrude deeper into holes 12 in theretaining block 8. Since the condition is cumulative and occurscyclically, it is not desired that the compressive force becomecontinually greater as the buckets grow circumferentially, as would bethe case with a resilient spacer. This would merely aggravate thecondition and would cause a gradual increase in compressive stress.Since compensating device 7 exactly compensates each time for thecircumferential growth, the average tightness between bucket bases 3 ismaintained at a substantially constant level.

FIGS. 4-6 illustrate a second modification of the invention. There, thecompensating device 7' is made up of four retaining blocks 13 withcrushalble pins 14 separating a pair of blocks 13 on each side of therim 4. Each of the retaining blocks 13 has projections 15 fitting inspecial grooves 3b in faces 3a on the separated bucket bases. Eachretaining block 13 also has holes 16 extending partially therethroughwhich perform the function of holes 12 in FIG. 3.

FIG. is a circumferential view showing the pair of retaining blockmembers 13 on eitherside of the turbine wheel. FIG. 6 illustrates thateach pin 14 terminates in a frusto-conical tip 17 which fits into hole16 in block 13. The operation of the modification of FIGS. 4-6 issimilar to that of FIGS. 1-3, in that the conical tip 17 of pin 14extrudes into hole 16 when the compressive force on pin 14 'becomes suchthat the local yield point of the pin is exceeded. Therefore, themodified compensating device 7 becomesshorter in its circumferentialdimension an incremental amount as the compressive force due todeposition or oxidation between bucket base interfaces 6jis cyclicallyrepeated.

It will be apparent that in some cases, the retaining blocks 7, 13 ofthe two modifications shown might be omitted and the adjacent bucketbases used to perform this function. That is, holes 12, '16 would merelybe formed in two separated bucket bases with the pins extendingtherebetween.

While the invention has been described with particular 60 reference to aturbine wheel, it will be apparent that the invention is also applicableto any member having a number of segments disposed thereon with planarfaces tightly abutting one another where the faces are prone to separateduring some conditions of operation to allow deposition or oxidationtherebetween. However, it will be apparent that the arrangement isprimarily useful in a wheel where the planar faces are radial, wherebyan increased compressive stress between the segments can become relievedby yielding ina direction normal to the compressive stress, i.e.,radially. By designing the pins properly, so that the predeterminedcompressivestress at which they yield is always less than that at whichthe wheel would yield radially to relieve this compressive stress, theprob lems heretofore caused by arch binding are significantly reduced.

While there has been described herein what is at present considered tobe a preferred embodiment of the invention, and one modificationthereof, it may be understood that various other modifications may bemade, and it is intended to cover in the appended claims all suchmodifications as fall within the true spirit and scope of the invention.

What we claim as new and desire to secure by Letters Patent of theUni-ted States is:

'1. In a turbine rotor, the combination of a rotor wheel having a rimdefining c-ircumferentially extending dovetail ribs, a plurality ofturbine buckets disposed on said rim each including base portionsdefining dovetail grooves mating with said dovetail ribs, said baseportions defining radial faces on opposite circumferential sides thereofwhich abut those of adjacent base portions and which are subject tooxidation and scale accumulation when cyclically exposed to a motivefluid, and means separating the bases of two spaced adjacent buckets,said separating means comprising a first member defining a recess and asecond member disposed in the end of said recess, said first and secondmembers being disposed between said spaced buckets so that acircumferential closing movement thereof will extrude the second memberinto said recess and permanently shorten the circumferential dimentionof said separating means under a predetermined circumferentialcompressive force.

2. In a turbine rotor, the combination of a rotor wheel having an outerrim, a plurality of turbine buckets held on said rim, each bucketincluding a dovetail base portion efining opposite radial faces tightlyabutting adjacent bucket base faces, said faces being subject toincremental cumulative growth due to oxidizing or scaling properties ofa motive fluid upon temperature changes thereof, and means separatingthe bases of two spaced adjacent buckets, said separating meanscomprising at least one member defining a recess, and circumferentiallydirected pin means having portions slightly larger than said recess andconstructed so as to extrude into the recess under a predeterminedcircumferential compressive force between bucket bases, whereby theseparating means will shorten permanently in a circumferential directionto compensate for the circumferential growth of the bucket bases.

3. In a turbine rotor the combination of a rotor wheel having an outerrim, a plurality of turbine buckets held on said rim, each bucketincluding a dovetail base portion defining opposite radial faces tightlyabutting adjacent bucket base faces, said faces being subject toincremental cumulative growth due to oxidizing or scaling properties ofa motive fluid upon temperature changes thereof, and means separatingthe bases of two spaced adjacent buckets, said separating meanscomprising retaining block means engaging one bucket base, saidretaining block means defining circumferentially directed holes, and aplurality of pins having ends engaging the other bucket base and havingtheir other ends disposed in the retaining block holes, said pins havingportions slightly larger than said holes so as to extrude into said olesunder a predetermined circumferential compressive force betweenbucketbases, whereby the separating means will shorten permanently in acircumferential direction to compensate for incremental circumferentialgrowth of the bucket bases.

4. In a turbine rotor, the combination of a rotor wheel having an outerrim, a plurality of turbine buckets held on said rim, each bucketincluding a dovetail base portion defining opposite radial faces tightlyabutting adjacent bucket base faces, said faces being subject toincremental cumulative growth due to oxidizing or scaling properties ofa motive fluid upon temperature changes thereof, and means separatingthe bases of two. spaced adjacent buckets, said separating meanscomprising first and second spaced retaining block means each engagingthe base of one of said spaced buckets, said first and pressive forcebetween bucket bases, whereby the separatsecond retaining block meanseach defining circumferening means will shorten permanently in acircumferential tially extending holes in line with one another, and adirection to compensate for the circumferential growth plurality of pinshaving tapered portions on opposite ends of the bucket bases.

and extending between said first and second retaining 5 block means anddisposed in said holes at either end References Cited in the file Ofthis Pawnt thereof, said tapered portions extending only partiallyUNITED STATES ATE into said holes so that they will be extruded deeperinto said holes under a predetermined circumferential com- 894659Kauberg July 1908

1. IN A TURBINE ROTOR, THE COMBINATION OF A ROTOR WHEEL HAVING A RIMDEFINING CIRCUMFERENTIALLY EXTENDING DOVETAIL RIBS, A PLURALITY OFTURBINE BUCKETS DISPOSED ON SAID RIM EACH INCLUDING BASE PORTIONSDEFINING DOVETAIL GROOVES MATING WITH SAID DOVETAIL RIBS, SAID BASEPORTIONS DEFINING RADIAL FACES ON OPPOSITE CIRCUMFERENTIAL SIDES THEREOFWHICH ABOUT THOSE OF ADJACENT BASE PORTIONS AND WHICH ARE SUBJECT TOOXIDATION AND SCALE ACCUMULATION WHEN CYCLICALLY EXPOSED TO A MOTIVEFLUID, AND MEANS SEPARATING THE BASES OF TWO SPACED ADJACENT BUCKETS,SAID SEPARATING MEANS COMPRISING A FIRST MEMBER DEFINING A RECESS AND ASECOND MEMBER DISPOSED IN THE END OF SAID RECESS, SAID FIRST AND SECONDMEMBERS BEING DISPOSED BETWEEN SAID SPACED BUCKETS SO THAT ACIRCUMFERENTIAL CLOSING MOVEMENT THEREOF WILL EXTRUDE THE SECOND MEMBERINTO SAID RECESS AND PERMANENTLY SHORTEN THE CIRCUMFERENTIAL DIMENTIONOF SAID SEPARATING MEANS UNDER A PREDETERMINED CIRCUMFERENTIALCOMPRESSIVE FORCE.